Conventionally, in a multilayer circuit board having arranged therein signal lines, a power line and a ground line and having arranged the surface thereof an IC elements, a LSI element and circuits, a serious problem is posed by the fact that with the increase in speed and density, the unrequired radiation is liable to occur due to high harmonics which have an effect on other devices.
The unrequired radiation is roughly divided into two types, the common-mode radiation caused by the resonance due to the potential fluctuations of the power layer and the ground layer and the radiation of differential mode caused by the signal line layers and the component parts mounted. In the prior art, various methods have been proposed to reduce these unrequired radiation.
A method generally employed for reducing the radiation of differential mode is by shielding, and a method specifically employed is by coating a conductive paste containing a resistance material on the surface of the board.
In order to send a signal to the circuit board from an external source, a transmission line such as a coaxial cable is connected by a connector with an external signal source. Such a connection is schematically shown in FIG. 13.
In this diagram, the signal source is designated as a transmitting terminal unit 100 and the circuit board receiving signals from the transmitting terminal unit 100 is designated as a receiving terminal unit 101, with a coaxial cable 102 connected between them. The circuit board constituting the receiving terminal unit 101 is connected to the coaxial cable 102 by a connector not shown. The transmitting terminal unit 100 is also connected with the coaxial cable 102 by a connector not shown.
In the transmitting terminal unit 100, an outward line 100a connected to a signal source 100c for generating a pulse-like signal of frequency .omega.j and voltage V0 is connected to an internal conductor 102a of the coaxial cable 102, and an inward line 100b is connected to an external conductor 102b of the coaxial cable 102, each by a connector not shown. Also, the receiving terminal unit 101 is equivalently expressed by a receiving line 110a, a return line 101b and a load impedance ZL connected between them. This receiving line 100a is connected to the internal conductor 102a of the coaxial cable 102, and the inward line 101b is connected to the external conductor 102b of the coaxial cable 102, respectively, by a connector not shown. The inward line 100b of the transmitting terminal unit 100 and the return line 101b in the receiving terminal unit 101 are grounded, and the external conductor 102b of the coaxial cable 102 is used as a grounding line.
In this configuration, a signal line is formed of the outward line 100a from the signal source 100c of the transmitting terminal unit 100, the internal conductor 102a of the coaxial cable 102, the receiving line 101a, the load resistor R, the return line 101b of the receiving terminal unit 101, the external conductor 102b of the coaxial cable 102 and the inward line 100b of the transmitting terminal unit 100.
In this signal line, the signal output from the signal source 100c in the transmitting terminal unit 100 is sent to the internal conductor 102a of the coaxial cable 102 as a voltage V1a and a current i1a, and received at the receiving terminal unit 101 as a voltage V1b and a current ilb, respectively. Also, in the return path of this signal line, a signal of a voltage V2b and a current i2b flows from the receiving terminal unit 101 along the inner surface of the external conductor 102b of the coaxial cable 102. Not only that, the current is reflected by an equivalent impedance at a junction point B between the coaxial cable 102 and the receiving terminal unit 101, so that the current leaks out to the outer surface of the external conductor 102b of the coaxial cable 102. This flows as a leakage current i3b along the outer surface of the external conductor 102b of the coaxial cable 102. The signal flowing along the inner surface of the external conductor 102b is input to the transmitting terminal unit 100 as a voltage V2a and a current i2a. The current is also reflected by an equivalent impedance at a junction point A between the coaxial cable 102 and the transmitting terminal unit 100. As a result, part of the current i2a leaks out to the outer surface of the external conductor 102b of the coaxial cable 102 and flows along the outer surface of the external conductor 102b of the coaxial cable 102 as a leakage current i3a.
The coaxial cable 102 forming this signal line has a resonance point of a wavelength .lambda. satisfying the relation L=(2n-1).multidot..lambda./4 (n: a positive integer) where L is the length of the coaxial cable 102. Therefore, as long as the wavelength of the currents i3a, i3b flowing along the outer surface of the external conductor 102b of the coaxial cable 102 is sufficiently away from the wavelength .lambda., the currents i3a, i3b which are originally very small pose no problem. In the case where the wavelength of the currents i3a, i3b is proximate to the resonance point of the coaxial cable 102, however, the coaxial cable 102 develops a resonance, operates as a mono-pole antenna, and thus generates an unrequired electromagnetic radiation. Let the length L of the coaxial cable 102 be 1 m, for example. A resonance point occurs at a resonance point of frequency equivalent to odd multiples of f=3.times.108/4.times.1 =75 MHz.
The leakage current described above could be eliminated, if the case of the interior of the transmitting terminal unit 100, the interior of the coaxial cable 102 and the interior of the case of the receiving terminal unit 101 could be completely hermetically closed by integrating the case of the transmitting terminal unit 100 completely with the outer surface of the external conductor 102b of the coaxial cable 102 and also by integrating the case of the receiving terminal unit 101 completely with the outer surface of the external conductor 102b of the coaxial cable 102. Actually, however, such a configuration is substantially impossible to realize. Therefore, the occurrence of the unrequired radiation described above is unavoidable.
In view of this, according to the prior art, in order to suppress the unrequired radiation, a ferrite core 103a called a common mode core or a common mode choke is arranged on the side end of the transmitting terminal unit 100 of the coaxial cable 102, and in similar manner, a ferrite core 103b is arranged on the side end of the receiving terminal unit 101.
The provision of the ferrite cores 103a, 103b is equivalent to the insertion of a series circuit including an inductance and a resistor in the signal line along the outer surface of the external conductor 102b of the coaxial cable 102 due to the inductance and the polarization derived from the ferrite cores 103a, 103b. It follows, therefore, that the leakage currents i3a, i3b flowing along the same outer surface are suppressed. The absolute value of the impedance of the ferrite cores 103a, 103b is conventionally set to about 100.OMEGA. from the viewpoint of the material and structure.
According to the above-mentioned conventional method, however, a conductive paste is coated on a comparatively flat portion of the surface of the board but cannot be coated on the component parts mounted or the portion where they are mounted. Even in the case where the board surface is shielded by the conductive paste, therefore, the shield layer is opened in the portion where the component parts are mounted, and the unrequired radiation leaks out from the opening and the unrequired radiation (common mode radiation) occurs anew due to the resonance at the opening. The unrequired radiation thus cannot be suppressed sufficiently.
Also, the unrequired radiation from the transmission line such as a coaxial cable connected to the board, as explained with reference to FIG. 13, can be suppressed to some degree, but not necessarily to a sufficient degree, by arranging a ferrite core at the ends of the transmission line. In the foregoing description with reference to FIG. 13, the absolute value of the impedance of the ferrite cores 103a, 103b is set to 100 .OMEGA.. If 100 .OMEGA. is not sufficient, however, the absolute value of the impedance is increased by arranging a plurality of ferrite cores 103a and ferrite cores 103b. In this way, the effect of suppressing the leakage currents i3a, i3b can further be increased at the sacrifice of the requirement of using a bulky, heavy ferrite core. The use of a plurality of ferrite, on the other hand, is equivalent to the coils wound in a plurality of turns, between which the electro-static capacitance may occur, thereby posing the problem of the occurrence a new resonance.
A more critical problem is that even when a quality ferrite material is used for the ferrite cores 103a, 103b, the permeability .mu. thereof has such a frequency characteristic that the frequency of 300 MHz or higher sharply reduces the permeability .mu. and makes it impossible to produce a sufficiently large impedance. At such a frequency, the ferrite cores 103a, 103b have a lesser effect of reducing the leakage currents i3a, i3b, and are unable to suppress the unrequired radiation generated from the coaxial cable 102.
In the case where the clock frequency of the signal involved is as low as about 10 MHz, for example, it is sufficiently lower than 300 MHz and therefore it is possible to use the ferrite cores 103a, 103b with a sufficiently large permeability .mu.. Thus, the fundamental wave and triple harmonics of the problem leakage currents i3a, i3b causing the unrequired radiation can be sufficiently suppressed, and the frequency characteristic of the permeability .mu. is not a serious problem. In recent years, however, the clock frequency of the personal computer or the like has further increased to not less than 100 MHz or not less than 200 MHz, etc. With this high clock frequency, the permeability .mu. of the ferrite cores 103a, 103b is decreased to such an extent, at the fundamental wave and the triple harmonics of the signal, that the effect of reducing the unrequired radiation cannot be exhibited.
An object of the present invention is to provide a low-EMI circuit board which obviates the above-mentioned problems and is capable of effectively suppressing the radiation of mainly the differential mode.
Another object of the invention is to provide a low-EMI cable connector which is both compact and simple in configuration and can effectively suppress the unrequired radiation in the signal transmission line.